text:
Why does the radar dome on the E-3 have a white stripe down the middle?
Every picture I can find of the Boeing E-3 Sentry has a blue/black radar dome on top with a fairly thick white stripe down the middle but I can't figure out why.  Is it perhaps related to the position of the radar unit inside?  Why is it marked like this?

I couldn't find a specific reason why they used different material colors, but this AWACS Surveillance Radar brochure from Northrop Grumman describes the features of the radar system including the rotodome. The black parts are the electromagnetically transparent radome, and the white parts house the electronics.

@greghewgill you should put that as an answer.

The white strip is the metallic part which houses the electronics and serves as support structure for the antenna. The black part is basically a composite (fiberglass) shell, which covers the antenna. According to RAF:

The radar is actually the white part of the disc, the black being there to balance and streamline the radar as it rotates

According to this page:

Each AWACS 707 plane has two fiber glass radomes, joined by a 6 ft. (1.82 m) wide metal spine to form a 30 ft. (9.14 m) diameter radome.

This can be seen in this Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome.

The Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome. (Credit: Scott Carson. Twitter.). Image taken from aviation.stackexchange.com
The fiberglass shell is painted over in black as black was one color that did not require metallic particles in its paint composition (which would interfere with radar waves) as discussed in another question in aviation.stackexchange.

and as a beneficial side effect, you can now see when it's rotating which can be more than beneficial if you're a maintenance guy crawling around on top of the aircraft on the ground (yes, the thing can rotate without the radar active).

Wouldn't it be simpler to have the radar head rotate inside a fixed fairing dome (like a traditional land-based radome), rather than having the head attached to the dome and then having to rotate the entire assembly?

Slight correction.  What you're seeing in that picture (on the left) is just the s-band antenna.  Behind it is the antenna support electronics.  The rest of the radar (transmitter, receiver, processor, etc) is located in the cabin area.

During aircrew briefings for operations with AWACS, it was stated that aircraft in the vicinity of an AWACS aircraft can determine that the radar is operational if the dome is rotating, easily seen with the white stripe.  Not a good idea to fly too close to an AWACS with its' powerful radar emitting EMF.  Any material component can be coated in any color material so the dome could easily be all white or all black; even red or blue.


text:
How do engineers make sure UAV don't land on their front propeller?
Take, for instance, the following UAV:

and suppose you have a payload with a very big volume that fits exactly in the fuselage (and there is no more space), but is extremely light. Thus, the heaviest part of the UAV becomes the propeller and its engine.

How do engineers make sure the UAV stays balanced and doesn't land right on its propeller?
Do they add some weights somewhere near the rudder and elevator to keep its center of gravity somewhere near the wing's trailing edge?

Thanks.

@jjack as you can see the UAV I posted above doesn't land using landing gear or anything like that: https://youtu.be/1hAWmmYUODU?t=39 . So I guess there must be another way to make it land safety? This video is not so clear, but actually it somehow glides down, lands on its belly and slides a bit on the ground.

@jjack does that mean belly landing is impossible for uavs whose prop is at the front (like on the image)?

@jjack as an EE, i should be able to make a system that gives a variant amount of current to the prop in order to make it turn gradually slower to land. But I am affraid it may tip over and land right on the prop when it turns too slowly. So for extra security (as it s the first time I am building a foam uav) i d like to make it land on its belly. Any idea s how I could achieve tat and eventually what sources I should read in order to do that?

At least in some cases, a net is used to capture the UAV.

@LandonZeKepitelOfGreytBritn: I think one point you're still not understanding is that landing is not the problem. Flying is the problem. If the plane is too nose heavy it will not fly and would simply crash on the propeller on take-off. I would never call crashing on take-off "landing". The only way to make the plane fly is to rebalance it by adjusting weight distribution. Because of this we don't need to consider landing conditions since it's impossible to fly with the nose being too heavy.

@jjack I am a bit late to the party but here is a link of what I was talking about, ie a nice belly landing: https://youtu.be/xl91tKg126Q?t=114
Unlike the animation I shared above, this is a real movie where you see the UAV slides nicely on its belly and lands in a very safe and smooth way, which is what I am trying to achieve. Do you still think being able to land in such a way is all about a constant pull on the elevator and making the prop turn slowly or are there some other things I should take into consideration?

Due to center of gravity limitations, it would be impossible for any type of aircraft to be loaded so that the front became the heaviest part. The aircraft would not be able to fly.
These UAVs are meant to crash land and DO land on the front propellor. You can do a Google search and see multiple videos showing the “break apart” landings they are designed to do.
Here are some landing videos:
Puma landing 2
Puma landing 1
Coast Guard Conducts First Unmanned Aircraft System Deck Landings

The "Puma landing 2" link goes to the same landing as "Puma landing 1".

Link fixed. Thanks

For the Facebook Aquila, they recently added a mechanism to align the propellers horizontally so they don't totally wreck.  From their second test flight report:

Aquila's second test flight took into account the lessons we learned from our first flight. In advance of the second flight, we incorporated a number of modifications to Aquila, including:

Adding “spoilers” to the wings, which help to increase drag and reduce lift during the landing approach

…

Installing a horizontal propeller stopping mechanism to support a successful landing

This, unfortunately, didn't work all that well.

A few seconds before landing, the autopilot stopped the propellers as planned in order to lock them horizontally. The propellers are meant to lock in the horizontal position to avoid damaging them when touching down. In this flight, the motors all stopped, but only one propeller locked horizontally.

Still, the article says that the aircraft was in great shape aside from some minor damage that occurred during landing.

Wow, I wonder how they implemented this horizontal locking. Did Facebook make their own motor controllers which allow powering certain coils within the motor in order to put it into a certain position?

@Michael Not sure, but in the original Aquila post (https://code.facebook.com/posts/268598690180189/flying-aquila-early-lessons-from-the-first-full-scale-test-flight-and-the-path-ahead/), they talk about the need to, "develop more efficient onboard power and communication systems" as a future challenge.  I don't know if that means they have to go from-scratch or if they've done that, or what.

I don't see how this answers the question about the nose being to heavy compared to the tail. The question is: "Do they add some weights somewhere near the rudder and elevator to keep its center of gravity somewhere near the wing's trailing edge?", not whether they lock the propeller. As already commented, the problem is more how to fly than how to land.

@mins It doesn't directly, I was just adding another example answering the broader question of not-wrecking-on-landing.

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What if all passengers pay to check in extra baggage?
Suppose a given flight is fully booked and everybody shows up. Not only have they all brought the allowed fully loaded carry-on & checked-in luggage weight but exceed the free-limits. They are all willing to pay extra for, say, an additional 10 Kg per person. Let's assume average weight for all passengers.

Will this cause the flight to go over the Maximum Take-Off Weight? If so, will the flight depart?
What are the measures taken for the passengers & their cargo for such flights? Are they divided to take separate flights?
What if everyone has 20 Kg extra? How about 30 Kg? Is there any limit to it?
Does it add if everyone is overweight?

I am asking this in a general context of a non-budget airline.

I had a bad experience a few weeks ago. I was on the same flight as a wheelchair sport club. Their extra equipment put the aircraft overweight, so the airline removed an arbitrary set of suitcases, mine included, and put them on a later flight. Southwest was too disorganized to scan the tags, so we all had to line up and have them copy the tag numbers.

Related: How does a commercial airliner measure its weight/mass?

Some airlines have embargo restrictions limiting the number of bags you're guaranteed to be allowed to check, even if you're willing to pay, when the flight being overweight is a common occurrence. (For instance airlines with high seating density like Spirit have these on long routes https://customersupport.spirit.com/hc/en-us/articles/202096466-Are-there-any-restrictions-on-how-many-bags-I-can-bring-)

Related: Travel SE: Can I get a discount on my flight for not being overweight?

Also related: Aviation: How is a 436 seater A330 fuel efficient?

@CarlfromBusbud and others have a limit on the number of extra bags checked total per flight. So For example there's a max. of 50 extra bags for the flight, the 51st person with an extra he wants to check in is out of luck.

To answer the questions as asked,

We won't depart over the maximum takeoff weight. We'll do something to bring the takeoff weight down to not exceed the limit.
Generally, people fly as scheduled if at all possible, and as much as possible their bags go on the same plane that they do, and other cargo is a lower priority. There are exceptions; passengers might be denied boarding when the plane is simply too heavy with every one and every thing on board, but this is a last resort and not particularly common.
Total weight is total weight, and the more of it there is, the more limited your options become.  This is explained in more detail below.
If the people are overweight, it's probably not a factor in the numbers, because for scheduled commercial flights, most airlines use average numbers for passengers and their carry-on bags.  So it doesn't matter if they're all skinny or all heavy, they all count for the same "200 lbs".  For military charters and sports charters, actual weights are used, and some non-US carriers may use actual weights as well.

Okay, that's the what; let's discuss the "why":
There are a lot of "it depends" elements here, but in most cases, what happens is that the amount of other freight that can be carried gets reduced, and in the worst case some of the bags stay behind.  But that's probably fairly uncommon.  It's a very worst-case scenario when you can't even take all the passengers -- pretty rare, in my experience.
Your maximum takeoff weight is limited by one of three limits:

a takeoff limit (how much the airplane can weigh to take off & climb out while meeting all of the various requirements),
an enroute limit (essentially, if you lose an engine at some/any point enroute, can you clear all of the terrain along the way to get to a suitable landing site), and
a landing limit (your expected landing weight is no greater than the maximum allowed landing weight).

In practice, the enroute limit is pretty rare, and if the departure runway is long enough, the landing limit is usually the most restrictive.  So, most of the time, your limit boils down to: basic weight of the aircraft, plus cargo, plus passengers, plus landing fuel, can't exceed the maximum certified landing weight.  The aircraft weighs what it weighs, and a full load of passengers is essentially a given weight (most flights use an "average" weight value rather than actual passenger weights).  So the interesting variables are the landing fuel, and the cargo weight (bags plus freight).
If it's a good weather day and no alternate is required, then the required fuel for landing is one number -- let's say 5,000 lbs in a 737, assuming a fairly "best case" sort of day.  (Various operators may have slightly different numbers, but this is a high-level explanation, not way down in the details.)  But if the weather is bad, that number can go up: allowing fuel for holding, as well as for a possible diversion to an alternate.  Depending on how bad the weather is and how far away the alternate, minimum landing fuel might be 8,000-10,000 lbs for a 737, or more.
Thus... my maximum landing weight is 144,000 lbs for a 737-800.  If the airplane and the people weigh 125,000 lbs, I have 19,000 lbs left over.  If my landing gas is 5,000, then I can take 14,000 lbs of bags/cargo/whatever -- which is a lot.  If the weather is crummy at the destination and instead my min landing gas is 12,000 lbs, then I'm limited to 7,000 lbs of bags.  If today's flight had 6,000# of bags and 3,000# of cargo, it all goes in the first case, but not in the second, and it's usually easier to let the freight take the next flight than the bags -- it gets expensive to deliver bags that the airline didn't get on the same plane as the passenger!
There can also be instances where the max takeoff weight is more limiting... a high & hot airport and/or short runways can limit how much weight you can take off with.  So perhaps "today" where I'm departing, the best that I can do (after considering current winds & temperature and the various runways available) is a max takeoff weight of 165,000#.  If I'm burning 25,000# of gas getting where I'm going, then I'll land at 140,000# -- max landing weight (144k) is no longer the limit, max takeoff weight is.  And the computation now looks at the various weights plus min TAKEOFF fuel compared to the max takeoff weight, rather than weights plus min landing fuel compared to the max landing weight.  But the basic process is essentially the same.
Okay, disclaimer, there are various other things than can come into play that I've glossed over for the sake of simplicity -- max ZFW and improved climb and wet landing runways and plenty of other stuff.  If you're studying for your dispatcher license, this is NOT the answer to address all your what-if's.  It's just an overview.

Dispatchers are licensed?

@JeffreyBosboom Yep (in the US). There's an FAA exam and everything.

So if you're unlucky and there's a fat guy convention you didn't know about, could you take off something like 100lb per passenger heavier than the average you're assuming? Would you notice as the passengers filed on that they were all like 280lb plus carry-on and wedging themselves into the seats, prompting you to call the ground crew to take some freight off? Or are the rated weights in any case generous enough that an unusual event like that still doesn't matter?

"...14,000 lbs of bags/cargo/whatever..." Holy cow that's 7 tons of cargo. How many suitcases is that (or the 7000 lbs)?

@BruceWayne - If everyone packs to the 50 pound limit, that's only 280 bags, or 1.5 bags per person in a single-class 189 passenger 737-800.

@Johnny - I never really thought about it, but makes sense

@SteveJessop: Not hypothethical, unfortunately : Air Midwest Flight 5481. So really the first claim "We won't depart over the maximum takeoff weight." is not 100% true. It's unlikely that this crash was the only occurrence, statistically many more planes must have taken off overweight.

@MSalters -- I'm not sure if it'll ever become practical to weigh-in all passengers and carryons for mainline flying, which is what would be needed to have a 0% chance of taking off overweight.  (It's also true that smaller aircraft are more sensitive to small changes in weight than larger aircraft are)

I would have thought that the landing gear can report the weight it is supporting, at least approximatly.

I also surprised that the landing gear doesn't have load cells that report the weight.

@SteveJessop For most passenger airliners, even the situation you describe wouldn't affect things too much. It would matter a lot more on a small turboprop like the one MSalters mentioned, though. I did the calculations for what would happen if everyone's weight doubled on an A330-300 in my answer to this question on travel.

@MSalters many more did, one result of that crash was to increase the FAA average weight for passengers their bags by quite a bit. What made this flight crash where others survived was a maintenance error that hadn't been noticed during inspections.

Realistically what about a football team or something?  I could see a sports team having a much higher average weight.

@dalearn Sports charters use actual passenger weights, not average values.

@RalphJ but what about if they were on a passenger flight?

@JeffreyBosboom I can't speak to today's licensing of dispatchers, but back in the 1970s when I got my ATP, the dispatcher's exam was the exact same exam as the ATP exam. This was back before there was a separate ATP exam for Part 135 and 121. It was the one exam that I actually paid for a multi-day class that essentially taught the test. The class members were mostly pilots preparing to sit for the ATP class, but there were some preparing to sit for the dispatcher's class. At the time, the airplane used for all the performance questions was the 727 as I remember.

@Terry not sure about today, used to be dispatchers had to hold an ATP themselves. It was a good job for older pilots who'd lost their medical.

@jwenting For what it's worth, I found the 2017 Airline Transport Pilot, Aircraft  Dispatcher, and Flight Navigator Knowledge Test Guide at https://www.faa.gov/training_testing/testing/test_guides/media/FAA-G-8082-1.pdf. On page 2 it has the statement "The aircraft dispatcher applicant is not required to have the flying skills of an airline transport pilot, but  is  expected to have the same knowledge."

@Terry yeah, so they don't have the hours requirements

The situation you described actually happened recently in flights to India from the Middle East. Many bags were simply left behind and carriers were making arrangements to transport them at the earliest. Reasons ranged from avoiding overweight and also the inability to reduce fuel being carried due to monsoons (implying more hover times) in Southern India.

Scores of Indian expatriates in the Gulf who came home for Eid found
  their baggage left behind due to the massive festival rush.
Executives at some of the Indian carriers said a majority of
  travellers coming home for the festival had packed heavy bags and were
  even willing to pay excess baggage charges. But due to load penalty
  issues — not going beyond the maximum take-off weight of an aircraft
  without offloading passengers or carrying less fuel — meant that bags
  had to be left behind
A senior Air India executive said this issue was faced mainly with AI
  Express flights that operate between Gulf and south India. "Passengers
  were flying with lots of heavy baggage. Aircraft were carrying more
  fuel to factor in diversions due to bad weather (heavy rains) or
  longer than usual hovering times at airports. Due to this bags were
  left behind and we are making all attempts to ensure that they reach
  the passengers at the earliest," the executive said.
Source: http://economictimes.indiatimes.com/articleshow/53109304.cms

The answer so far suggests everybody gets a seat. A customer manager of an passenger airline once told me that's not always the case (I won't mention names here, but this is a famous international airline).
Not everybody who bought a flight ticket will get a boarding pass.
Now, first thing first: no sane pilots will depart with an aircraft that is overweight. An overweight landing might damage the landing gears and require inspection and repair. In the worse case, the capabilities of the aircraft is exceeded (climbing ability / structural limit / flight controls authority etc.) and you crash.
Unless all passengers show up precisely at the same moment, and all travel as a very big group, people are bounded to arrive at the airport, some earlier, some later. Even if they all get to the counters at the same moment, some passengers will check-in first while others wait at the queue. Now, when do we know the weight of the aircraft? After everybody has checked-in? Nope~ with computers, the weight of all checked-in passenger + bags + cargos is known the moment they're checked-in. So the result would be, those who arrive at the airport relatively late will be denied a boarding pass to the flight although there are still empty seats in the cabin.
If anybody has travelled on an "ID pass" (a super-cheap flight ticket sold only to airline's employees and their families), you'd know these passes are always the last to know whether they get a seat. The ground crew have to wait until check-in is closed, then check the standby passenger list, cargo list, standby cargo list (yes there are standby cargos!), only after that will they start considering ID pass travelers. The point here is, cargo carry revenue as well, and while it is easy for the ground crew to deny a cargo box boarding the flight, it's not so easy for the business people to explain why my important package for the international exhibition was delayed 8 hours when my company bought a flight ticket for my package which your airliner promised would get it there.
In the end, it's easier to turn away those who arrived at the airport relatively late. Some airliners overbook flights. That's one reason you can't get on the flight - there are no more empty seats in the cabin. Weight is another reason - there are seats, but no more lifting capability from the aircraft. Always arrive for check-in early!

Hmmm... I would still think it would be an incredibly extraordinary circumstance for baggage to be chosen over a ticketed passenger when an aircraft is at its weight limit. Honestly, I've never even heard of that happening for any normal-size airliner. I could believe it more on very small regional jets or turboprops, but even then it would be extremely rare. Denied boarding for overbook situations is fairly common, but not so much for weight.

@reirab not baggage to be chosen, but cargo to be chosen. Especially on a large aircraft like 747 or 777. The ground crew almost never tell the px it's weight - it's difficult to explain. It's easier to just tell people they're too late, since under this scheme the ones who don't get a seat are the late ones, so it's fairly easy for people to accept.

A lot of cargo isn't guaranteed, and even if it is, cargo is often cheaper per pound than the IDB penalty (e.g. up to $1350 to lose ~200 lbs). There's actually an IATA cargo priority order for issues just like this, I recall top of the list is human organs for transplant, then AOG equipment, and goes to place perishables like food before general cargo.

The maximum takeoff weight is an important number. These are the consequences of additional weight on the aircraft.

Slower acceleration on takeoff -> more runway length to reach the same speed.
Slower deceleration if the takeoff needs aborting -> more runway length needed to stop.
Higher stall speed -> aircraft needs more speed on ground to get airborne, at high altitude with thin air the stall speeds become dangerously close to the operating speed.

Flying a commercial aircraft over weight is a never event. The kind of mistake that should never occur. The consequence of doing so is that it turns a minor event that pilots can manage into an incident that the pilots have no safe options.
That said, on one occasion, on a small aircraft in India, the weight was reduced on the aircraft by removing fuel, flying half way to a small airport and then adding more fuel to complete the journey.

The flight will not depart overweight.  In fact, there are a number of factors that can make the max take-off weight variable, including local weather.
Passengers and Bags would probably be kept together where possible, for customer service reasons.  It would cost more to reunite bags with passengers than to bump other cargo (freight).  One or two extra bags per passenger would not break max take off weight for most commercial airliners.  Extra bags on smaller aircraft can definitely be a factor.
Passengers aren't weighed as they board the aircraft.  1 Passenger + Bag generally has a fixed "standard" weight for doing Weight and Balance calculations.  Something like 120 kg/Passenger.
The standard weight for Passengers is meant to account for variation.  At some point, overweight passengers may be required to buy an additional ticket and use two seats.

In the context of a non-budget airline, it is important to consider that many (Most? All?) non-budget airlines operate freight service with leftover capacity on their flights.  Freight gets bumped often due to weight limitations, but generally this is acceptable given the levels of service available in Air Freight.  Things that are heavy or bulky cost much more to ship via air at a guaranteed delivery service level.  Freight can wait!

I think the assumption here is that the airline guarantees the carriage of the bag on the same flight but this is not true. Ref United's policy A.8 in below link
https://www.united.com/ual/en/sg/fly/contract-of-carriage.html#tcm:76-6642
In the event of a foreseeable situation where bags may be more than normal for a flight (eg sports teams, bands), the load-planner may decide to allot more weight/space for the bags. If this is a last minute increase then cargo may be offloaded to release more weight for the bags.
Under certain route conditions, there may not be much cargo and if the aircraft has already been fueled there may not be weight available for the bags, as de-fuelling is a tedious and wasteful procedure it is quite likely that the bags will be left behind.

There are at least two possible weight limits on every flight. One is the aircraft's MTOW (Max takeoff weight). Another is determined by the weather, altitude, and runway length: is there enough runway for a safe takeoff? Is there enough obstacle clearance after takeoff, for an aircraft that loses an engine during takeoff?
The weight is estimated by: Empty weight (specific to the aircraft) + cargo + passengers + fuel. Passenger weight is approximated by a formula (although as Americans have gotten fatter, the formula needs tweaking).  So the answer to #1 is: "It depends on how far the flight distance is. For short flights, with less fuel, even a very full aircraft is unlikely to violate a weight limit."
No legal pilot will take off with an aircraft that is overloaded. So the answer to #1b is "No, the flight will not depart until something is changed."  In the scenario you outlined, they have several choices:

Turn away some passengers.  Only if they figure out the problem in  time, before loading is completed.
Offload some luggage at the last  minute — as described in previous answer.
Reduce fuel load. The only way to do this is to plan on an intermediate stop, which will make everyone late. So this is unlikely.

Good pilots and airlines will try to anticipate problems. On a hot day with a long flight, where the runway length could be a constraint, I expect that they will hold some people on a standby list. Just before departure, they can make all the needed calculations and decide whether to take some or all of the standby passengers. (I don't have personal knowledge of this procedure.)
In San Diego where I live, when the wind changes direction it can decrease the allowed weight, because taking off then requires a different runway. There is a hill near the end of the second runway, which is high enough to reduce the weight limit in some situations!

"No legal pilot will take off with an aircraft that is overloaded." At least in the GA world, overloaded aircraft are a major contributing cause to crashes.

@RonBeyer "GA world"? Sorry for my ignorance, but google only suggest Georgia, which I'm sure is not what you intended.

@Lenne General aviation. Wikipedia sums it up as "all civil aviation operations other than scheduled air services and non-scheduled air transport operations for remuneration or hire" and offer as actual examples of what it is among others "flying clubs, flight training, agricultural aviation, light aircraft manufacturing and maintenance".

If take-off weight is the concern you could pack on JATO packs until it isn't.

@Joshua: ...except that civilian aircraft do not generally use RATO.

I work as a ramp agent for a major airline so I can tell you exactly what will happen in this type of scenario.
Once a checked bag gets weighed, very rarely will we get more than 5 heavy bags for an A320 or A319 type aircraft with 177 passengers max in any one plane and on average we only get 2 heavy bags out of an average of 72 checked bags daily for a full 177 passenger flight.
But! Let's play with your little fantasy of absolute perfect storm of probability here:
If ever! Ever! We get 177 Heavy! bags to load onto an aircraft no one I mean no one! Will load them. These little demon bags we call heavys are responsible for throwing the backs of many ramp agents causing serious spinal injuries that is notoriously common in this occupation.
Honestly the entire occupation reminds me of how reckless humans can be. I work in an airport where people board using an Ada ramp (outdoors boarding) so I see people all the time spitting out their gum on the turmac, littering, smoking near a plane, going under stanchions, ropes, and even jumping over fences to get on the wrong plane as though we create obstacles for them to board! I swear if I get 8 truck tires and lay them down near a plane as to create an obstacle course we will always have 2 to 3 dumbasses a day sprint through it while quickly darting their foot through each tire as though it were a real obstacle course.
So anyway it felt like I threw out my back on that last paragraph so "back" on track here: (pun intended)
The most bags we loaded onto an aircraft at one time was 130 bags with 5 heavys and let me tell you something: God forbid your out of shape when you have to load these bad boys because the pain is coming. Loading each of these bags onto a plane consists of 3 people: one person putting the bag on the conveyor belt, one person receiving the bag on top within the belly of the aircraft (feeding bags) who then slides the bags to the third who is responsible for lifting and stacking the bags neatly against the bulkhead (back wall) however! Sometimes! we are understaffed like at the airport that I work in where there is only one person in the belly receiving the bags and let me tell you my friends, even if you are a cross fit athlete at an Olympic event there is no preparing for this one for it is the ultimate ungodly workout with many having passed out in the bin from an exhaustion. Yes we have had heart attacks while carrying your bags. Vastly underpayed we are dying for your ability to fly so basically like the military we are sacrificing our lives in exchange for your freedom to see your families. God bless us.
So if ever we get 177 bags we will need 5 people to load these whereas on average we only have 2 ramp agents available.
The great news is that they actually have people like me in the world who will load all 177 heavy bags by myself if need be and I can do it all in under 30min. If you don't believe me share this and I will do it as a spectators event.
Realistically a thirdparty carrier will come into assist like fed ex or ups etc.
Hope this helps.

This doesn't appear to be clearly answering the question and the various issues it raises. At the start it says if you get 177 heavy bags, “no one I mean no one! Will load them.” However, at the end you're suggesting what is required to load them and that you have people like yourself who will do it, which appears to be contradictory. Past that though, what if nobody will load them? What happens logistically? Do some bags get loaded? What happens with the ones that don't? What if you do load them all (since you're also suggesting that might happen), how does the flight manage that?

Great questions sorry I wrote this quickly. The plane has a weight and balance limitation or capacity of how much weight is needed to create lift and if the plane to include fuel, passengers, bags, and basic weight of the aircraft to include commissary items and crew is over that set limitation then weight would be taken off the aircraft with bags being the easiest way to get the job done.

Randomly selected bags will then get sent to an office where shipping accommodations will be made

Also if we all refused to load bags it will be treated like a strike for any organization and accommodations like negotiations would be held in getting the rampers to load. It will take Third party companies 3 weeks to get clearance and credentials to load the bags leaving it to management like the stations manager and supervisors to load the heavy bags themselves in which they would most likely do.

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What powers an aircraft prior to the APU being switched on?
I know that APUs are power plants for non-propulsion related purposes and that they are turned on before takeoff.
What powers an airplane prior to the APU being turned on?

Don't rush to accept, usually waiting 24 hours is recommended so all timezones can see and answer the question.

Onboard batteries for DC and a ground power supply for AC.
The ground supply can come either from an airport vehicle or from the stand itself.
Since the standard AC in aviation is  115V and 400Hz, the usual ground power supply needs to be converted to a higher frequency. This used to be done with a rotating converter, i.e. a motor (at 110V and 60Hz or 220V and 50Hz) driving a generator (at 115V and 400Hz), but can probably be done with solid state electronics these days. As for why the 400Hz, there is a good answer here but it boils down to: higher frequencies can work with lighter transformers, but also radiate more, so 400Hz was chosen as a compromise.
Of course, on more advanced aircraft with weight to spare, one can find ways to convert DC into AC and viceversa. DC can be converted to AC using an inverter, while the oppoosite is achieved through a rectifier (typically the rectifier is only one component in something more elaborate like a switched-mode power supply, since by itself it would give a pretty choppy "DC")

The batteries can also provide AC, via the on-board inverters.

On smaller business jets, Ground Power may be in fact DC rather than AC.

Other than the APU, there are multiple ways to provide electrical power to an aircraft:

Battery: The battery is typically the first thing you would turn on and it usually provides DC power to emergency systems only (at least on an airliner, smaller aircraft are fully powered by the battery). Running only on battery power will however deplete the battery quickly. The battery can also be used to start the APU.
Ground Power: Most (maybe all) airliners can accept a connection to a Ground Power Unit (GPU), which will supply AC electrical power to run all aircraft systems on the ground. These can be either mobile units (typically powered by Diesel):

(source)
or direct cable connections to the airport power grid:

(taken from this question)
Engine: This would be the method of last resort, but you can run an engine to provide electrical power via its generator. Note however that starting a jet engine would typically require bleed air from the APU or an Air Start Unit (ASU). Some turboprops can run an engine with a stopped propeller to provide power (Hotel mode). The engines also provide all electrical power during the flight, which is also used to re-charge the battery.

Running an APU on the apron is typically discouraged or restricted1 by the airport because of the noise and pollution. It does however provide one advantage over ground power: the APU also supplies bleed air to the air conditioning systems. Without it, air conditioning is only possible via a Pre-Conditioned Air (PCA) hose. See What is this tube connected to a 757 for? for details.
The APU is usually switched off after starting the main engines. It can however be used in the air on some aircraft. See In what conditions is the APU used in midair? for details.

1 Some airports have restrictions on when an aircraft is allowed to run the APU, e.g. Amsterdam:

The use of aircraft APU is forbidden in these stands in the period between 2 minutes after blocks for the arrivals and 5 minutes before off blocks for departure.

The APU will still be used to start the main engines during push-back.

Then DC batteries are recharged during flight?

Yes, whenever AC power is available (either from ground, APU generator or engine generators), it will be used to charge the battery.

What's the plug look like ?!

@Fattie That's a nice question, you should aks it.

@Bianfable - thanks, I did !

… on the other hand some airports have shortage of ground power supplies, so they just let everybody use APU all the time and act really surprised when ATR crews ask them to bring the GPU, because they only have Hotel mode and it's reeealy inefficient.

Worked at an airport for 3 years.  Aircraft were either powered by battery, GPU, jet bridge, or engines.  But being powered by engines presented a problem, in that all the blast zones would be deadly and severely limit what work can be done on the aircraft, if any.
Generally the procedure was to get the ground power connected as soon as the aircraft was parked.  Alternatively, the pilots could use their APU, but that costs them fuel, and it's less expensive for them to have us use our fuel (though I'm sure that cost is passed into the parking fee the airline pays for each gate.)  Also, working on an aircraft that had an APU running was extra loud, even with hearing protection.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the night effect?
I read the night effect takes place in radio navigation, especially VOR and NDB. My questions are:

What is its nature?
Why does it occur?
What are the consequences of it?

The Wikipedia page for NDB navigation has a pretty good explanation of it. It is also called "Twilight Effect".

It does not just affect radio navigation, it also affects AM radio.

In short
The night effect is actually a mirage. Listening to radio communications originating from below the radio horizon is like looking at a looming:

Looming optical illusion, source: SKYbrary
Both are strange things, unless we force ourselves to take into account electromagnetic waves (including light) don't travel in straight line but are subject to regular refraction.
UV and X-rays from the sun create layers of ionized atoms in high atmosphere. Some of these layers have refractive properties, and one acts as a wall preventing waves from the ground to reach the refractive particles.
The night effect is the casual result of a favorable timing: There is a window after sunset where the blocking layer disappears, and the refracting particles are still active, albeit less active than on daytime:

Ionosphere activity changes at sunset
Attenuated waves are redirected towards the ground during this period, possibly to a receiver located further than the horizon. In this phenomenon, ionized particles play the same role than hot air for looming.

Frequency spectrum overview
As your question involves distinguishing several ranges of frequencies, let's make a brief summary of the RF spectrum. Radio frequencies (RF) are a small part of the whole electromagnetic spectrum, along with light and ionizing rays at work in the ionosphere:

RF themselves are divided into "bands" which properties are more or less different. It's sometimes impossible to settle whether a color is green or blue, or yellow or orange, the same is true for these bands, the limits are really not important in most cases.

Wave propagation modes
RF waves propagate using different means at the same time:

Space wave (electromagnetic "line of sight", i.e. some refraction)
Ground wave (mostly induced currents)
Sky wave (large ionospheric refraction).

Space wave vs sky wave vs ground wave
The night effect has its origin in the sky wave, so if you are not interested you may skip the two next sections.
Space wave
Neutral atmosphere (below about 100 km) shows a gradual change in air refractive index, refraction constantly decreases with altitude. The consequence is the space wave is refracted and doesn't travel in straight line (Huygens-Fresnel wavelet principle), as often approximated.
This curvature is about the same for all frequencies in the RF range. When calculating the radio horizon for regular free space propagation, the standard is to use a radius for Earth which is actually 33% larger than the actual radius (4/3 k factor). With this radius, the line of sight can be represented as a straight line.
Ground wave
The ground wave propagation encompasses reflection on ground and obstacles, diffraction on obstacles and horizon, and refraction thanks to current induced in ground.
Because of the refraction and diffraction, this mode of propagation tends to follow Earth curvature, but it mostly affects LF and MF, higher frequencies (e.g. VHF and UHF) do not really take advantage of this propagation.
In ground propagation the wavefront is slowed down where it is closed to the ground. As the wave propagates perpendicularly to its wavefront, this more vertical wavefront can propagate further at ground level without creating shadows.

Ground wave: Wavefront slowed down by ground
Communication distance is increased. However as the ground wave progresses the effect is less and less significant. Vertically polarized waves (NDB and HF voice communication) induce stronger ground currents and propagate better than horizontally polarized waves (VOR variable signal).
The ground wave propagation, like the direct wave propagation, occurs any time, day and night. The third mode is the one of the night effect.
Ionospheric layers and sky wave
Atom ionization consists in losing or capturing one of more electrons, and becoming electrically unbalanced. Natural ionization occurs in the ionosphere under the bombardment of solar ionizing rays which detach electrons from atoms. This takes place in the F-layer at about 300 km, and in the E-layer at about 100 km. Waves can be affected by these layers when they are active.
A layer of ionized atoms makes an efficient refractive material for radio waves. However the lowest D-layer blocks waves during the day because of the ionization of heavy nitrogen monoxide $\small NO$) absorbing wave energy as they vibrate under the effect of the wave. This absorption decreases at night when the source of ionization disappears.

Reorganization of ionospheric layers at night
The higher layers are ionized more and more longer than the lower ones, and ionization persists more in the higher layers. The combined effects of D, E and F layers leads to:

An absorption of LF/MF/HF frequencies by the D-layer at daytime, preventing them to reach the E and F layers able to refract them.

A refraction at night and twilight when E-layer and above all the F-layer, where free electrons recombine more slowly, and some ionization is maintained overnight, are still active and the D-layer has ceased blocking waves.

The sky wave often proceeds by hops between ionosphere and earth surface, as earth surface, particularly ocean water, is an efficient reflector:

Sky wave reflection on the ground and skips
There is a significant attenuation as the wave bounces back and forth, and in practical the signal is too weak to be used after one or two hops, unless the link is optimized for such operations.
Because the sky wave proceeds by hops, reception is favorable at specific distances from the transmitter and is characterized by minimums and maximums spaced along the direction of propagation, the distance between maximums depending on the frequency.
Sky wave extends significantly the range of LF/MF frequencies. Sky wave impact is less noticeable on VOR and DME range, even if there are exceptional / sparse episodes of VHF refraction by the E-layer (sporadic E). Past VHF there is no noticeable sky wave.
Formally the maximum usable frequency varies with solar activity and is given by the secant law.
As the propagation of NDB frequencies is augmented at night by the sky wave, a NDB can be detected at a greater range by a receiver. This is also true for other mode of communication in the same range of frequency (e.g. voice communication in HF over the oceans).
Because low frequencies are the only ones subject to sky wave propagation, over-the-horizon (OTH) radars use them instead of SHF.

Over-the-horizon radar. Source
Fading
The sky wave propagation is associated with a slow variation, the signal can disappear and reappear a few seconds later. This is due to the instability of the refraction, the heterogeneity of the atmosphere layers, and the recombination of waves after a multi-path trip and a rotation of the polarization. This fluctuation is globally known as fading.
Errors introduced by the sky wave
While sky wave propagation is in general a good thing but, like any other multipath phenomena, it also has negative effects: It changes the polarization of the waves and the direction of the wavefront. As ADF works by detecting the direction of the signal wavefront, a refracted NDB signal introduces inaccuracy in the ADF indication.

The beacon seem to move, and the bearing is not reliable. Some cross check is necessary to confirm the validity of a bearing.

What an amazingly well-researched answer. Excellent.

@BurhanKhalid: Thanks for your kind feedback, your generous gesture and your chivalrous spirit. I invite everybody to read your profile.

Thank you for the great answer. Can you elaborate on your point that "Electromagnetic waves are not entirely understood"? It was my impression that modern Electrical Engineering and physics have had a pretty much complete understanding of electromagnetism (especially standard EM wave propagation) since Maxwell's Equations were devised in 1862. Is your comment about meteorological earth phenomena in particular?

Great answer, but EHF stands for extremely high frequency, not extra high frequency.

@Sean: Thanks. We are both right, EHF n. and adj. extremely or extra high frequency;  (a) n. a radio frequency in the range 30 to 300 gigahertz (OED), though there are more entries in Google with "extremely". Personally I prefer extra for consistency with ultra and super. Those prefixes are used without consistency across disciplines anyway, e.g. voltages.


text:
Why are the in-flight refueling probes of French-built fighter aircraft non-retractable?
French-built fighter aircraft seem to have in-flight refueling probes that are not retractable, as seen from the linked images below. What are the advantages of a non-retractable refueling probe that make it worth the increased drag?

Rafale, and Mirage 2000.

That refuelling probe seems to create a large blind-spot in the pilot's view; http://vignette3.wikia.nocookie.net/acecombat/images/1/12/MIR-2000-5_cockpit_view.jpg/revision/latest?cb=20110923003027. I find it surprising they wouldn't move it off further to the side

According to Dassault, this configuration was chosen in order to reduce complexity and therefore avoid deployment/retraction problems:

Failure-prone systems have been eliminated early on in the design process:
... and the refuelling probe is fixed in order to avoid any deployment or retraction problem.

There are some other advantages as well:

A non retractable probe takes up less space in the forward fuselage (where space is at a premium) compared to the retractable one. For example, when the F-16 was fitted with a retractable probe (Conformal Air Refueling Tanker/System (CARTS)) for the Indian MMRCA competition, it had to be done via a change to the forward section of the right conformal fuel tank.

Modifications for retractable probe in F-16; Lockheed Martin photo , image from f-16.net
Another good example for this is the French Mirage F1s supplied to Libya, which had their radars removed to accommodate the retractable probes and the Mirage F1C-200 version, which necessitated the lengthening of the fuselage to accommodate the (non-retractable) probe.
The French aircraft, especially the Rafale are not exactly known for their large noses. in such cases, it is better to have a fixed probe, thereby saving the space required for the deployment/retraction mechanism.

It weighs less as the system is simpler- basically, you are eliminating the deployment/retraction system of the probe.

This system offers increased fuel flow and reduces maintenance requirements.

One thing to note is that the Mirage 2000's refueling probes are removable (it appears like Rafale's too is removable), so the drag penalty is not there when the mission doesn't call for it. At the same time, this reduces the weight penalty. Interestingly, it means that the number of probes can be less than that of the aircraft itself, with the probes installed as per requirement.

Another thing is the French doctrine- Unlike US, the Armée de l'Air  doesn't put too much into mid air refueling or stealth requirements.

It can also be interpreted as a design choice- all the French aircraft, right from Mirage F1 have had fixed refueling probes (while some were modified to accept retractable ones).

@ymb1 It was implied (by Eurofighter) that the nose size was one of the reasons for having the side stick, rather than center one in Rafale.


text:
Other than a swing wing, what types of variable geometry have flown?
We're well familiar with swing wings, on small and large aircraft from both the east and west.
What other types of variable geometry have been tried?

Perhaps too mundane or not "geometry", but don't under-estimate the change between flaps retracted/ cruise configuration, and flaps extended/landing configuration on any modern airliner.

And there's the Wright Flyer from 1903,  which used wing warping controls to do what later airplanes did with ailerons.

Assuming you mean by-design as opposed to more spontaneous variable geometry ?

@nigel222 Good point, it could easily be argued that movable flaps and slats are variable geometry. They're mundane like you point out, but only because they're so good.

Any aircraft is a variable geometry aircraft if you fly it fast enough.

No time to answer, but Barnes Wallis Wildgoose and Swallow deserve a place here.

@JohnDvorak  ouch!

Do Estes models count? They do fly.

Are you looking for anything in particular?

@Harper UAVs, large and small, are fair game. I don't know which models you mean, but as long as the variable geometry is intended to improve flight characteristics I think it's fair.

@CrossRoads This question is to learn about the subject. The bounty is only because I don't think it's fair to select a single arbitrary answer as "correct", since they all are, so instead I thought I'd award bounties. I just didn't realize I have to wait a day to award the bounty, so maybe that was a bad idea.

Somebody flagged this question as "too broad," but it looks pretty specific and clear to me, so I'm voting to leave it open.

The Tu-144 had retractable canards, used for good control during landing and takeoff, but retracted during cruise for better streamlined shape.

Edit: I will take "variable geometry" to mean a big change in the wing or lifting surface, or thrust vectoring. This is how I understand the term.
If you took it literally, then a whole host of things come up like retractable landing gear, drag chutes, drop tanks... I don't think this is what the OP had in mind, but correct me if I'm wrong.
And let me just point out the caveat, that conventional control surfaces (ailerons, rudders, elevators) are very much variable geometry in the literal sense. From an aerodynamics point of view, they can be thought of as changing the camber of the surface, which deflects airflow and produces a certain amount of lift.
Now on to the continued list:

All-Moving Tails (Stabilator). This is where the entire tailplane (horizontal stabilizer) changes its angle, rather than just an elevator. Many fighter aircraft have these, like the F-16.

Variable Shock Cones, like in the SR-71. These moved backwards or forwards depending on flight speed, to give better shockwave intake geometry to the engine.

Also, the interior of the SR-71 engines themselves had variable geometry. Parts of the flow shut off so that it became a ramjet, and at low speeds, parts opened again to act more like a turbojet. I don't know the exact details.

Thrust Vectoring. There's all kinds of thrust vectoring. Some don't even involve variable geometry, but most do.

Tiltjets. Someone mentioned Tiltrotors, but not tiltjets. They seem to be much rarer.

Variable Pitch Propellers. There is definitely some varying geometry here, but not sure if everyone would call this thrust vectoring. But if thrust is a vector, and a vector has both direction and magnitude, then I'd say it qualifies.

Certain kinds of gliders. In hang-gliders, the pilot can move his weight around to control direction. The wings also flex in response to this. In paragliding, the pilot pulls on ropes that adjust the geometry of the "sail", controlling direction to a certain extent.

I have seen other experimental stuff, e.g., this pdf, but they seem to be unmanned so far. Not that that disqualifies anything, but being man-rated or put into production is a big milestone.

Oh, and how could I forget ornithopters!

Ornithopters are machines that actually flap their wings like a bird, providing lift and thrust all in one.

Great collection, showing that actually all aircraft have variable geometry.

The single piece elevator was actually the key to going supersonic and maintaining stability. The supersonic shock wave would render the conventional hinged elevator ineffective. Discovered by Yeager and Ridley, by manipulating the variable incidence fixed portion of the elevator on the X1, and finding that elevator effectiveness was maintained in the transonic speed range. This morphed into a single piece elevator, the 'flying tail'. Not an issue on delta wings, because the shock wave isn't near the balance portion of the elevons.

Please provide the sources of your images.

Extending wings
The idea is to have a larger wing area at takeoff for more lift and a smaller wing area in flight for more efficiency.
The NIAI RK and followup RK-I used two tandem wings that served as rails for an extendable panel that could be rolled out between them.

The project failed because Stalin was so enthusiastic about it he had it use the most powerful engine available which was too unreliable. I can't find a reason why the concept was not tried again.
The German FS-29 glider of 1972 had a different arrangement. It had an outer wing that fit over an inner wing and could unsheathe like a sword. Only one was built.

"I can't find a reason why the concept was not tried again." Too much added weight for the gain?

@Mast given the success of the RK I'd venture that the weight was worth the performance gain at the time. I'd speculate that much higher interest in jets sucked up resources for projects that had a much lower potential performance. Also availability of more powerful engines and longer runways means less need for high lift devices. But again, speculation.

I'll add that my supervisor (Prof. Cheeseman of Southampton University, UK) worked on telescoping rotor blades - I don't think it ever flew though.

@Mast Added weight is no big deal in gliders, nowadays many single seater have a 40 kg engine sitting behind your back. If telescopic wing system fits regulations and gives serious performance advantage racing pilots would be rather sure about what to trade the 40kgs of the engine for

Oblique Wing
A normal wing at takeoff then pivots during flight. High-risk idea to make transport aircraft more efficient in the transonic regime.
Lighter and simpler than a swing wing, with no change in the center of lift as the geometry is changed. Disadvantage is that flight characteristics become asymmetric left and right, plus problems with rigidity. See this question for more.

You should combine this with your other answer.

Please, as AEhere says, merge your two answers. If you don't know how to do it, I can do it for you.

@Federico they're two separate answers, right? Shouldn't we allow users to vote on them separately and differently?

@Federico it is my understanding that providing two answers is allowed on this site, and does not require policing.

@JJJ no, it's a question that requires a list, all items from the same author should go in the same answer. Theoretically the question could be closed as too broad, or should have only one community wiki answer.

@Koyovis not for cases where the answer is the same and is listing two different elements of the same list

@Federico is that a rule on this site? On other sites (e.g. ELU), it's encouraged to post them separately so they can be voted on separately to get a better order of answers based on quality, recent edits, etc. See this question on the general meta site.

@JJJ https://aviation.meta.stackexchange.com/a/1642/1467 the point, also on the general meta, is <when you have two distinct answers>. Two elements of the same list are not distinct answers

@Federico without making this into a big discussion, I'd say two elements in a list are distinct as one element can be more relevant or useful than another. It's not a closed yes or no question where to list items contribute as reasons to the same yes or no answer. As you say, by that reasoning all could be in the same community-wiki answer but I don't think that improves readability and usability for future visitors because different list elements cannot be voted on separately. ;)

@JJJ you're welcome to join us in chat to discuss this. I still do not see how elements of the same list are not the same answer. particularly when it is a self-answer

this answer and related question are being discussed on meta: https://aviation.meta.stackexchange.com/q/3845/1467 and https://aviation.meta.stackexchange.com/q/3846/1467

In addition to types already mentioned by other answers:
Tiltwing
Was used to allow VTOL operations by tilting the entire wing, as can be seen on the Hiller X-18. The concept was never used outside testing as far as I can tell.

Aeroelastic Wing
Tested on the X-29 and later on the Boeing X-53, which was based on the F/A-18 Hornet. The idea here is that the wing can be twisted to control roll, giving better control while reducing load on the aircraft. Only used in testing so far.

Canard Rotor/Wing
The concept was that an aircraft could use a rotary wing similar to a helicopter for vertical take-off and landing; once up to speed it would stop the rotor and use it as a conventional wing. Was never tested in VTOL mode and the project was cancelled. See Boeing X-50 Dragonfly for more information.

Variable geometry wingtip
The XB-70 Valkyrie had hinged wingtips which could be angled downwards by up to 65 degrees to improve lift and stability in certain regimes.

Is there a difference between your aeroelastic wing example and early wooden/fabric wings which would just deform the wing rather than have hinging ailerons?

If I've understood it correctly it's more a matter of how the wing is twisted. Early aircraft used control wires to twist the wing; the aerolastic wing uses a combination of control surfaces and aerodynamic load to twist the wing. NASA published a bit more information (https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-061-DFRC.html), where it appears that early F/A-18 had problems with wing twisting, resulting in the wing being strengthened. The X-53 restored the original more flexible wing and modified some of the control  surfaces and systems.

For variable geometry wingtip, the NASA is still investigating the benefits on a drone.

The X-18's full tiling wing never went anywhere, but arguably this was critical inspiration for the V-22 Osprey

Variable incidence, on the F-8 Crusader.

From the Wiki page:

The most innovative aspect of the design was the variable-incidence wing which pivoted by 7° out of the fuselage on takeoff and landing (not to be confused with variable-sweep wing). This allowed a greater angle of attack, increasing lift without compromising forward visibility.

Tiltrotor, on the V-22 Osprey
From the Wiki page:

A tiltrotor aircraft differs from a tiltwing in that only the rotor pivots rather than the entire wing. This method trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.

And of course: rotating wings! Pioneered by Juan de la Cierva for the autogyro, and Igor Sikorsky for the helicopter

Variable incidence wing looks just like an all-moving tail, but for the wing. Didn't know they ever made that!

The Concorde had a drooping nose to allow it to be very streamlined during flight but offer better low-angle visibility when taxiing, taking off and landing.
Nose down when landing:

Diagram showing up and down nose positions:

-1. Don't mean to be rude but I have to question the "variable geometry" in this case. If you take it literally for any part of the aircraft then you might as well talk about retractable landing gear, flaps and slats, ram air turbines, adjustable air intake ramps, reverse thrusters, or even drag chutes.

@DrZ214 Don't all those things qualify?  If not, where do you think the line should be drawn?

@DonHatch All of those things are literally "variable geometry", but usually that term refers to variable geometry in the wings or a lifting surface, and maybe thrust vectoring too.

@DrZ214 Well this is a question about variable geometry other than a swing wing. :-)

Variable Camber Wing
From the late 60s to the early 90s NASA tested experimental variants of their F-111s; at one period they were trying out a "Mission Adaptive Wing":

The second phase called transonic aircraft technology (TACT/F-111A)
  added an highly efficient supercritical wing and later the third phase
  applied advanced wing (Mission Adaptive Wing-MAW) flight control
  technologies and was called Advanced Fighter Technology Integration
  (AFTI/F-111A). source

The F-111 was already a swing-wing aircraft, but this modification was a supercricital mission-adaptive wing with smooth variable camber, similar to the aero-elastic wing already mentioned.
Flight research concept can be read here and results can be read here.

source In flight - compare with landing wing below.

source

My dad worked on this program!

Never heard of this one, thanks for the report links!

If I'm seeing this correctly, it's effectively flaps and slats, but the wing itself bends down instead of having separate pieces droop. Or, to put it another way, as the flaps/slats extend/droop, they remain covered by the main wing skin instead of having their own skin and multiple sections.

Going waaay back to the Wright Flyer, aircraft were originally controlled by warping the wing. Literally yanking on the edges of the wing to twist it and induce a roll. In a few years this technique was replaced with ailerons on rigid wings.

Yet another extending wing design, the Makhonine Mak-10.

(cc-by-sa from FlightGlobal via Wikimedia Commons)

Common Variable Geometries
These are so successful to have become mundane.

Control surfaces. Rudder, ailerons, elevators, etc.
Movable flaps and/or slats. Increase lift (and drag) at low speed.
Spoilers and air brakes.
Retractable landing gear. Does change shape and performance of aircraft.
Drop tanks, drag chutes, bomb doors. Debatable, including for completeness.

Also rotating propeller- does that count?

@quietflyer I think that argument is very thin, but this CW is to keep the pedantry contained so edit it in if you believe in it.

Some early attempts at VTOL involved changing the vector of the thrust, not by tilting the engines or wings but by using other engines.
Doriner DO-31 used two Bristol Pegasus engines (same as the Harrier) for forward flight, and six smaller vertically oriented engines for vertical takeoff/landing. Originally, the Pegasus engines were to vector downward for takeoff/landing, but this was never tried. The DO-31 was canceled by NATO in 1970... the vertical engine pods increased drag to the point where useful payload was very low.
A successful effort to use vectored thrust in unique ways to increase lift is the Shin Meiwa PS1/US1 flying boat. Aside from the four turboprops, the PS1 also has a single GE T58 turboshaft engine mounted behind the cockpit to power a fan to blow air across the flaps for increased lift at very low speed for shorter takeoff and landing. A few years ago, a US1 landed in 15 foot swells to pick up a F16 pilot who had ejected over the ocean. China recently debuted what appears to be a copy of the Shin Meiwa, the AVIC AG-600. but it does not appear to use blown flaps. Its capability to deliver large numbers of troops quickly, without needing a runway, might come in handy should China decide to invade a certain large island.
The Boeing YC-14 transport also used blown flaps for STOL, by directing the output from the center mounted turbofans over the flaps. While the YC-14 never entered production, the very similar Antonov AN-72 did enter production, and has been quite successful.

Would a glider that can drop its motor below the fuselage for takeoff, then re-stow it for gliding fall into this category?

The engine, with electric starter for air starting, erects from and retracts into a bay in the forward fuselage by means of electro-hydraulic power.

I can't find a picture with motor extended.
https://web.archive.org/web/20110715225311/http://www.sailplanedirectory.com/PlaneDetails.cfm?PlaneID=330

Looks to me like the prop (not  motor )  is above ( not below ) the fuselage for launching--

Don't forget hang gliders.  In addition to the weight-shift (movable pilot) aspect mentioned in another answer,  modern hang gliders have a feature called "variable geometry" or "variable billow".  When the system is engaged by tensioning a cord or lever, the sweep angle of the leading edges is decreased by few degrees. The purpose is to increase the length of the trailing edge, as seen in plan view (from above).  This tensions the fabric of the whole wing, decreasing "billow" and twist (washout), which increases the L/D ratio and glide ratio and decreases the sink rate, especially at higher airspeeds, but at the cost of also making the glider less responsive in roll and thus harder to maneuver.  An additional side effect of most vg systems is a change in the anhedral angle of the leading edges.
You could also say that the "speed bar" system of a modern paraglider is a type of variable geometry.  As for that matter so are the basic steering controls of a paraglider which are a form of wing warping.
Also, a little-known fact is that the weight-shift roll inputs of a hang glider pilot actively pull the moveable "keel tube" (a structural element on/ near the glider centerline) in the intended direction of turn, thus actively warping the entire wing.  If you immobilized this tube and also sprayed the whole wing fabric with shellac to stiffen it in  a fixed position- even if while placed in a wind tunnel to "inflate" the fabric to an optimum shape- the glider would become extremely "stiff" and unresponsive to roll control inputs.  This in fact is the fundamental reason that engaging the VG system makes the glider harder to turn- the keel tube has less freedom to move from side to side.


text:
Do presidential aircraft use additional call signs?
From the answer to another question I learned that the call sign Air Force One (or Marine One, Navy One or Executive One) is transferred immediately to the aircraft currently carrying the president of the USA.
When president Obama visited the Netherlands (and probably on other occasions as well), I remember him arriving in one of two identical helicopters, to not give away his aircraft to would-be attackers. Using the call sign Marine One for either would defeat that purpose.
So are additional call signs used in such a situation?

I'd imagine "Marine One, flight of two" would suffice, but I haven't actually ever had a radio on me when a President is flying nearby!

So how would you know which one is which from the call sign?

@Simon if one is still circling while the other lands and it transpires from the ATC that the one circling is AF1.

That's not how they operate.  If you watch, they always fly as a pair.

@Simon presumably you aren't intended to be able to tell which is which. From an ATC perspective a flight of two is one aircraft. Communication among the military aircraft involved is a whole different question, and one we'd probably find difficult to answer here.

When multiple U.S. military aircraft are operating in formation, they do so as a "flight". This flight has a callsign, and in the case of Presidential transport flights, this callsign is the Presidential one, such as Air Force One or Marine One.
For POTUS transport using a fixed-wing aircraft, the flight typically consists of a single aircraft, usually one of the VC-25s specifically built for the purpose. Its fighter escort, if any (typically there is one), is a different "flight" that operates in tandem with Air Force One as the escort "element" of the complete transportation "package". Therefore, for Air Force One and for Navy or Executive fixed wing equivalents, the aircraft is the flight and has the callsign.
When the "Marine One" helicopters must be used, however, a very sophisticated "shell game" is played, as you already know. Not only are multiple helicopters used (I've seen up to 6 in use at the same time), they are externally identical in appearance, and they change position in the flight formation constantly. The Secret Service has even gone so far as to use body doubles that would make positive ID of the helicopter the actual President has entered difficult, even if an enemy faction had spotters in place to track each individual helicopter's position in the flight the whole time it's in the air.
This entire flight of multiple helicopters is what carries the "Marine One" callsign. Individual aircraft within the flight, if typical military radio procedures are followed, would be identified by adding a craft number suffix: "Marine One-One", "Marine One-Two", etc. I don't know whether the aircraft change craft numbers during flight to match formation position, or if the pilots just keep track of which chopper's where as they juggle.
So, a potential assassin would either have to take a 50-50 shot (or worse) at one of the choppers in the flight, or he'd have to coordinate with multiple launchers to attack all the choppers simultaneously (and the larger the effort, the more communication is required to plan and execute it, which increases the group's exposure and the potential for detection). If there is any reliable intel that the President would be targeted with this level of sophistication during a visit, he usually doesn't go; he has a VP, a Secretary of State and a cadre of ambassadorial representatives whose job it is to personally represent the President anywhere in the world the President himself cannot be, for any reason.

When the president travels, a TFR (VIP Movement) is issued. This ensures that other non-authorized airplanes are not in the vicinity.
The president generally does not travel to places where there is a chance of such insecurity. Although sometimes they take precautionary measures like as mentioned here:

On 8 March 2000, President Clinton flew to Pakistan aboard an unmarked
  Gulfstream III while another aircraft with the call sign "Air Force
  One" flew on the same route a few minutes later.

Executive One is also used as a call sign for the president.

When president Obama landed at the Museumplein in Amsterdam, there where two helicopters and according to the commentator, only at the last moment it became clear which one was carrying the president. You can say what you want about Amsterdam, but it's not that bad.

@SQB - You never know. The helicopters used as "Marine One" play this shell game every time they're used, because helicopters by their nature are slower and more delicate, and have less ability to evade or defeat a SAM or other ground fire.

Marine One is a flight of 2-3 helos even at home in DC, so a friendly country shouldn't feel slighted to see the same.  USSS's main job qualification is extreme paranoia, and given the number of US Presidents that have been killed, with good reason.


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A transfer function relating jet engine thrust to pitch angle during a phugoid motion
Crossposted on Mathematics SE

I am doing a school project that requires me to find a transfer function that relates the thrust of a jet engine (which could change with time in one way or another) and the aircraft's pitch angle during a phugoid motion. Phugoid motion is when an aircraft pitches and flies up along this pitch angle while maintaining constant thrust. The drag and the weight would slow the aircraft down, causing the pitch angle to decrease as lift--which depends on aircraft velocity along the flight path. Due to the lift being no longer able to counter the weight, the aircraft droops down, accelerated by gravity and its own thrust while speeding up. This increases lift and allows it to climb back up again. Then the cycle is repeated because all of the flight controls are down. My project's goal is to use fuzzy logic to increase and decrease the thrust so the pitch angle and rate of pitch angle change would be damped to 0.
A set of equations describing the phugoid motion: https://www.math.stonybrook.edu/~scott/Book331/Seeing_flight_path.html
The $\theta$ is the pitch angle:
https://www.math.stonybrook.edu/~scott/Book331/Phugoid_model.html
The way I see it, the acceleration/deceleration of the aircraft along the flight path can be found through:
$$a_{fp}(t)=T(t)-D(t)-mg\sin(\theta(t))$$
m is the mass of the aircraft,g is the gravity, T is thrust, and D is drag.
The velocity along the flight path can be found through
$$v_{fp}=v_{fp_0}-\int_{t_0}^{t_1}a_{fp}(t)dt$$
$v_{fp_0}$ is the initial aircraft speed along flight path (a given value).
The thrust is a time-varied input, the drag can be found through
$$D(t)=C_d\frac{\rho(v_{fp}(t))^2}{2}S_{wet}$$
Everything except vfp is known.
The lift of the aircraft is just like the drag:
$$L(t)=C_L\frac{\rho(v_{fp}(t))^2}{2}S_{wet}$$
The lift and drag equation here are all with respect to the inertia reference frame instead of with respect to the flight path.
The pitch angle should be
$$\theta(t)=\theta_0-\int_{t_0}^{t_1}\frac{\frac{L(t)-W}{m}}{\int_{t_0}^{t_1}v_{fp}(t)dt}dt$$
Where $\frac{L(t)-W}{m}$ is the vertical acceleration of the aircraft, $\int_{t_0}^{t_1}v_{fp}(t)dt$ is the displacement of the aircraft from $t_0$ to $t_1$ So this $\frac{\frac{L(t)-W}{m}}{\int_{t_0}^{t_1}v_{fp}(t)dt}$ is actually an angular acceleration. So we integrate this angular acceleration and find the change in pitch angle from initial $t_0$ to "final" $t_1$.
But I can't figure out what to do next, given that there are so many integrals.

The lift of the aircraft is just like the drag - no, I don't think so. There should be a phase shift between the two. Next, for thrust variation with speed it is important to know the bypass ratio. Did you try to model the airplane using finite differences? That avoids those pesky integrals ...

@PeterKämpf It is a school project, so I am going to simplify it and make thrust an input value of my Simulink system. I mean, it may be good if I can get a theta(t+1)=f(theta(t))

To perform such analysis the steps normally are: 1) write the equation of motion 2) linearize it via the small-perturbations hypothesis 3) do some simplifications according to the relative magnitude of each term in the equations 4) apply Laplace to change in the s-domain 5) from the transfer function get the searched values. Didn't your professor teach that? If not, any book about flight mechanics can guide you.

Why not hold vfp constant, since that's your goal.  Thrust damping could be based on Drag (now more or less constant too), so your fuzzy is only looking for +/- changes in velocity due to the gravity component of climb and descent angle.  It would then add or subtract engine thrust accordingly to help damp the phugoid.

For these kinds of control analysis and control synthesis, one typically uses a set of ordinary differential equations (ODE's). Phugoid motion (and Short term oscillation) are textbook examples for control of aircraft as these show the oscillation  frequency and damping of these modes very well. I will seek out the according equations as soon as I have time. From that point on you can then easily design an according control system.

@RobertDiGiovanni even when thrust is constant, the drag would change because the gravity would decrease the aircraft velocity, so the vfp would decrease as it climbs, then increase as it dives.

@RobertDiGiovanni yes, My project assumes and aircraft that lost all hydraulics and require thrust control technique--which is going to be accomplished by my program

@FaitoDayo please see Flight123 and Flight232.  Controlled flight is very difficult if hydraulics are lost.

@RobertDiGiovanni yes, this is exactly my premise. I am trying to emulate the Propulsion-Control Aircraft concept NASA developed by developing a everything-simplified version of this control system in MATLAB. That is why I need a transfer function

For control design, it is much more convenient to characterize the motion of a dynamic system via a set of Ordinary Differential Equations (ODE's) instead of directly trying to solve the equations... I will try to outline the "classical textbook" way of deriving and characterizing the phugoid motion of an aircraft which then can be used to design a control system. You can find similar derivations in most textbooks on aircraft dynamics and control, a free example is this one (although this might not be the best textbook on the matter).
For the analysis of the dynamic responses of aircraft, we first start with the well known (longitudinal) equations of motion of an aircraft. These are then linearized (a step which I am not showing). The resulting state space system in well known linear matrix equation form $\dot{x}=\mathbf{A}\cdot x + \mathbf{B} \cdot u$ is given as follows:
$$
\begin{bmatrix} \dot{u}\\ \dot{w} \\ \dot{q} \\ \dot{\theta} \end{bmatrix} = \begin{bmatrix} X_u & X_w & 0 &-g \cdot \cos(\theta_0) \\
Z_u & Z_w & u_0 & -g \cdot \sin(\theta_0) \\
M_u+M_{\dot{w}} Z_u & M_w+M_{\dot{w}} Z_w & M_q+u_0 M_{\dot{w}} & M_{\dot{w}}  \\
0 & 0 & 1 & 0 \end{bmatrix} \cdot
\begin{bmatrix} \dot{u}\\ \dot{w} \\ \dot{q} \\ \dot{\theta} \end{bmatrix} +
\begin{bmatrix}
X_{\delta_e} & X_{\delta_T} \\
Z_{\delta_e} & Z_{\delta_T} \\
M_{\delta_e}+M_{\dot{w}} Z_{\delta_e} & M_{\delta_T}+M_{\dot{w}} Z_{\delta_T} \\
0 & 0
\end{bmatrix} \cdot
\begin{bmatrix}
\delta_e & \delta_T
\end{bmatrix}
$$
The nomenclature is as follows: $u$ and $w$ are the forward and downward velocities, $q$ the rotational velocities and $\theta$ the pitch angle. Thrust and elevator deflection are given by $\delta_T$ and $\delta_e$. The derivatives like $X_u$ and $M_w$ are the aerodynamic/thrust terms derivated in respect to the different variables (therefore $u$ and $w$ in this example). The source (p. 83) also states how to relate these to more commonly known aircraft parameters here.
This fourth order system now mirrors two distinct aircraft motions:

The short period oscillation, therefore involving mainly $w$ and $q$
The phugoid oscillation, involving mainly $u$ and $\theta$

As you specified, you want to control the phugoid motion only with engine thrust (I pressume in reference to flight incidents like this one). Therefore you can neglect $\delta_e$ and also the coefficients $X_{\delta_e}$, $Z_{\delta_e}$, and $M_{\delta_e}$. To calculate the transfer function of the fourth order system, you can now either directly form the transfer function by calculating $H(s)=\mathbf{C}(s\mathbf{I} - \mathbf{A})^{-1}\mathbf{B} + \mathbf{D}$ , or we split the system according to the two different modes into two systems. Once into a system reflecting the short period oscillation
$$
\begin{bmatrix} \dot{w} \\ \dot{q} \end{bmatrix} =
\begin{bmatrix}
Z_w & u_0 \\
M_w+M_{\dot{w}} Z_w & M_q+u_0 M_{\dot{w}}
\end{bmatrix}
\begin{bmatrix} w \\ q \end{bmatrix} +
\begin{bmatrix}
Z_{\delta_T} \\
M_{\delta_T}+M_{\dot{w}} Z_{\delta_T}
\end{bmatrix} \cdot
\begin{bmatrix}
\delta_T
\end{bmatrix}
$$
and once into a system reflecting the phugoid motion (after some reformulation and setting $w=\dot{w}=0$, p 88)
$$
\begin{bmatrix} \dot{u}\\ \dot{\theta} \end{bmatrix} = \begin{bmatrix} X_u &-g \cdot \cos(\theta_0) \\
-\frac{Z_u}{u_0 + Z_q} & \frac{g \cdot \sin{\theta_0}}{u_0 + Z_q} \end{bmatrix} \cdot
\begin{bmatrix} u \\ \theta \end{bmatrix} +
\begin{bmatrix}
X_{\delta_T} \\
0
\end{bmatrix} \cdot
\begin{bmatrix}
\delta_T
\end{bmatrix}
$$
by neglecting all cross-coupling terms. It should be noted that by neglecting the cross-coupling terms, the accuracy of the system also diminishes. Therefore, if you can, you should avoid this, although it is easier to handle 2x2 systems directly.
Either way, I would strongly reccomend doing all of this numerically with tools such as matlab, which is very well suited for analyzing such systems, and then to also synthesize a controller.
Footnote: A much more convenient way of choosing the state variables is to choose the following state vector $x=\begin{bmatrix} V & \alpha & q & \theta \end{bmatrix}^T$, however sources of this choice are unfortunately closed source.


text:
Are there surveillance cameras in aircraft cargo bays?
I've heard that there are cameras on the fuselage and wings of some passenger aircraft. Is there a similar surveillance system in-built at the cargo bay/compartment of a passenger aircraft?
During the baggage loading / unloading of passenger aircraft, many burglary events can be experienced. I think that if there was a camera surveillance system, theft could be reduced significantly. At the same time, when the fire warning was received from the compartment, the pilot could confirm by monitoring the cargo compartment whether the warning was correct and avoid unnecessarily diverting. In addition, the perpetrators of compartment damage occurring in cargo loading / offloading could also be easily identified.
Why did the aircraft manufacturers not consider installing in-built cameras in the cargo compartment?

"the pilot could confirm by monitoring the cargo compartment whether the warning was correct and avoid unnecessarily diverting" It could be in a part they can't see, as a pilot I would divert regardless of seeing it on a camera or not. A little spark can quickly turn into an inferno. As soon as smoke filled the compartment, it would be useless. I'm also not sure they would thwart thefts (as rare as they are) because there are a lot of places to hide or steal before luggage gets to the aircraft.

If there is nothing to be seen due to smoke, a real fire warning has been confirmed via using the camera, isn’t it?

Right, but what has it bought you other than a distraction? You have an alarm going off and no reason to mistrust it. Why do you need the pilot, who is flying the plane, trying to coordinate an emergency landing, to also need to visually verify an alarm?

You need to have a camera that is very sensitive to smoke in order to detect small fires quickly... you want to do it by looking at light being absorbed or scattered by smoke... that device is exactly a photoelectric smoke detector which is what they already put in.

One good additional question would be "what is the purpose of cameras fitted on some aircrafts?" All those I know of (only the A380) are mainly here to help taxiing.

During the baggage loading / unloading of passenger aircraft, many burglary events can be experienced  [citation needed]

@RonBeyer High humidity can also cause a spurious warnings or the smoke detection system can create false alarm without an actual fire/smoke. This kind of fake indications can be accomplished with the use of integrated hold survelliance systems.

@GolfCharlie But how often do these happen that we would need an entire new system installed in every airplane? 1 in a million flights? 1 in 10 million? Doesn't the distraction/cost/complexity/weight outweigh the benefits of diverting the "one in a year" flight (even if it were per-airline) for a false alarm?

@RonBeyer The main benefit of the camera system would be to reveal who carried out damage in the cargo hold rather than using it to verify the fire. In plane damages found on arrival costs airlines a lot and it’s not an easy thing to find out who caused the damage.

I'm still not sure that is useful, but I won't spam this post with more comments about it. The damages that do occur in the aircraft usually occur during flight, not loading. Damages during loading are more bound to happen on the way to the plane, than in the plane itself.

Any burglary of cargo would happen outside the aircraft, after unloading the cargo. External cameras on the ramp are much more effective for this purpose. I don't know of any aircraft that has internal cameras just for the cargo area.


text:
Does anyone know what plane this is?
I have looked at hundreds of photos on line but nothing that has the upswept wings and the downswept rear wings with just one tail.  I'd like to see a picture so I can paint it properly - thanks

Pleasse update your question with a picture by using the edit link

Help us help you...  Is this an aircraft you read a description of, or have you seen one in person?  Do you have a photo?  Have you Googled "F-4" to see if that is the one you are thinking or?  If you don't provide us some feedback I am going to vote to close as too broad.

A fighter with "upswept wings and the downswept rear wings with just one tail"? sounds like an F-4.
BTW, "upswept" is called a "dihedral", and "downswept" is an "anhedral".

F-4 is pretty much the only plane fitting the description, allthough only the outer part of F-4's wings were "upswept".


text:
How does the G1000 differ from an airliner’s avionics?
I'm going through my multi-engine rating, and my flight training has always been on the G1000. I've had a simple system course on the CRJ-200.
In terms of IFR flying, I really don't see much difference between the two avionics in terms of procedural functionality, except for the FMS.
Anyone can point out what are some of the major differences between the two?
This question is only out of curiosity.

The CRJ's Rockwell Collins FMS-4200 as used in the CRJs is a 1980s design and is far "cruder" than the G1000.  You have a little control head with a small display screen and keyboard where you interface with it.  The G1000 is more like using any modern computer application where much is presented on the main screen with trackball or mouse selectable commands and data with a fairly intuitive presentation.  On the FMS-4200, you have keys on the side of the display to select options that appear next to each key, keys to select various modes on the control head, and you have a one-line "scratchpad" where you can type in text commands or other data.
You could say it's a bit like the difference between using the icon based Windows operating system and the older direct key entry MS DOS.  With a system like the 4200 you have to learn the selections and menus and keystrokes directly, instead of just knowing how to find your way around a pictorial presentation on a large screen that you use like any modern computer application.
If you've mastered the G1000 and you later find yourself on a CRJ type course having to figure out its 1980s FMS system, your G1000 skills probably won't help you that much and you'll just have to figure out the "old school" interface as something new.  To get though the type course in one piece, mastery of the FMS is critical, so if the opportunity to go on that course comes along, get hold of the training material as early as you can to get as much of a head start as possible.


text:
What is the correct procedure for opening the cockpit escape hatch on an ATR-72?
I’m hoping someone can help me understand the correct procedure for opening the cockpit escape hatch on an ATR-72.

As I understand it, the operator must hold the rear handle, and pull the forward handle to open the hatch, is this correct?
Does the hatch pivot open, or does it come off completely?
How would one climb out after opening the hatch?
How is the emergency rope used?
Thanks in advance.

I hope you are not asking this because your plane is on fire and you need to get out :p

@Bianfable Can't even get on a plane right now, let alone one that's on fire

The image belongs to an ATR 42-500. You can see the door opened on an ATR42 here

@Rsf is a -42 hatch significantly different from a -72 hatch in look and function? If not, then it probably doesn't matter much. If so, then maybe the OP can find a different picture.

@Rsf that photo is from an ATR 72-212A (I can confirm, not because I know much about the aircraft, but because I was the one who took the photo!) But it would appear that the escape hatches are not much different. From the reference you provided it looks as if the hatch detaches completely from the fuselage, and is passed out the top?


text:
How are duplicate call signs dealt with?
Many airlines (at least here in the U.S.) have flight numbers that use different aircraft for different legs of the flight. For example, Delta 158 uses a 747-400 from Incheon International in Seoul to Detroit, then an A320 from Detroit to Boston a couple of hours later. This got me to wondering what happens if an earlier leg is delayed such that it's arriving around the time that the next leg is departing the same airport? In particular:
Will both flights use the same radio call sign? That is, will they both be "Delta 158" (ignoring the "Heavy" difference in this particular case?)
If they do use the same call sign, how do controllers know which one is talking, especially if they end up on the same frequency at some point?
It seems like this sort of situation would happen somewhat frequently, at least here in the U.S. where it's common for a given flight number to include multiple legs and for those legs to operate on different aircraft.
This question is related, but doesn't actually address this particular question: Why are some ATC call signs different than the flight number?

Your question and the answers given are great - I had never considered the dilemma of Flight numbers. I used to own a Navion with 2FM in Alaska as the whole N number. Twice when I flew to the lower 48, I had a conflict. ATC simply informed us to use our WHOLE number. For me, I continued to use 2FM but the other aircraft had to bark out their entire N number. -  I learned something here - great question!!!

The scenario you're asking about is common.  Let's say that your Delta 158 from South Korea to Detroit is running several hours late, and the decision is made to operate the A320 DTW-BOS on time.  ATC doesn't accept having two aircraft flying with the same callsign at once due to exactly the sort of confusion you suggest, so something has to change.
What will happen is that the second flight -- i.e. the one that isn't already airborne, will get a new ATC callsign.  Sometimes you'll hear something like "Delta 158A" as the callsign of the flight, or they may use something entirely different like "Delta 9158" as the ATC callsign for that flight.  Each company will have their own method of resolving the conflict; some use letters, others don't.  Some can go to "9000 + original flight number," and others will simply use something like 9001 for the first "stubbed out" flight that day, 9002 for the next, and so on.
But, the basic answer is that the ATC callsign of the flight changes.
Which does NOT mean, incidentally, that the flight number as far as the passengers are concerned has changed.  It's possible to be "Delta 9158" or "Delta 9001" or "Delta 158A" or whatever for ATC and still be "Flight 158" when talking on the PA to the passengers.  The fact that the ATC callsign changed is transparent to the rest of the operation, since reservations, ticket counter, baggage handling, and so forth isn't upset when 158 ICN-DTW happens to overlap (in time) 158 DTW-BOS.  Only ATC worries about that, so the fix is only visible to them (and the pilots & dispatchers).

Are the pilots allows to just make up their own callsign?  Like "oh crap, 158 is taken, no problem... Chicago tower, Delta 069..."

Sadly, no. They are assigned by Dispatch, who knows ALL of todays callsigns. Because just because ATC doesn't have Delta 069 in the system NOW doesn't mean that they won't before you land, and the cascade of "oh that's being used" events would get ugly fast for a big operation like Delta!

@SpongeBob If the pilot could just make up their own call sign, it would be much more fun to use, say, Air Force One. :) Imagine the President's pilots' surprised when they call clearance delivery and are informed the call sign is in use. "Uhh... ok... I guess we'll have to use Air Force 1A..."

@reirab, wouldn't that be Air Force 9001?

In the case where they do rename a callsign (ex. Delta 158) to something similar (ex. Delta 158A), do they often issue a warning, such as "Warning: similar callsigns in use."?  I've heard that for planes with different tail numbers (ex. 87Sierra and 67Sierra)?

@AaronJPung ATC might do that if both flights ended up on the same frequency, although with one coming in (to, let’s say, Detroit) and the other going out, it would probably be a short time that they’d be talking to the same controller, if at all.

Who is ultimately responsible for the second callsign? Does the airline suggest it or ATC? If ATC how would they know each airline's policy with these overlap flights eg. use '9' prefix on flight number vs. 'add alpha character to flight number'? Australia used to use aircraft registrations as call-signs until a few years ago, which avoided the problem, but to match international standards, they now use flight numbers too, although the overlapping flight number issue is less prevalent there because the 'main' airlines stick strictly to their flight number ranges for domestic .vs intnl flights.

With GA here in Italy the standard is to always use your complete registration at first call (I-ABCD) and then the tower will eventually shorten it (I-CD) if that doesnt cause conflict. If they shorten it, you can too.

@Pete855217 Why don’t you ask that as a new question. Worth its own thread.